In recent years, a number of medical devices have been designed which are adapted for compression into a small size to facilitate introduction into a vascular passageway and which are subsequently expandable into contact with the walls of the passageway. These devices include, among others, blood clot filters which expand and are held in position by engagement with the inner wall of a vein, such as the vena cava. Vena cava filters are known in the art as described, for example, in U.S. Pat. Nos. 4,425,908, 5,669,933 and 5,836,968 and European Patent Office publication 0 188 927 A2, which are hereby incorporated by reference in their entirety. Such filters may include structure to anchor the filter in place within the vena cava, such as elongate diverging anchor members with hooked ends that penetrate the vessel wall and positively prevent longitudinal migration in either direction within the vessel. Such filters also may include structure to locate the filter within the blood vessel, such as near or along the centerline of the vessel. Such structure may consist of a number of locator members which press against the walls of the vessel with approximately equal force, thus causing the center of the filter to move to the centerline of the vessel. A filter including anchor members having hooked ends and locator members is disclosed in U.S. Pat. No. 6,258,026, which is hereby incorporated by reference in its entirety. Once the filter is positioned along the centerline, the anchor members can hook into the vessel wall in locations which help to hold the filter along the vessel centerline. Within a few weeks the endothelium layer grows over the anchors and locator members where they contact the wall, further holding the filter in place.
Blood filters which use anchor members as described above suffer from the disadvantage that the lateral force applied by anchor members to press hooks into the wall of the blood vessel continues to be applied even after the filter has been centered and anchored in place by the hooks and endothelium overgrowth. The continued application of force against the vessel wall by anchor members can lead to vessel damage or injury. Accordingly, there is a need for a blood filter that can secure itself in the vessel and reduce or otherwise remove the force applied to the vessel's walls after the filter is anchored in position.